In recent years, plasma display devices using plasma display panels (hereinafter, referred to as “PDP” or “panel”) receive attention as color display devices that implement a large screen size, thin body, and a light weight in displaying color images for computers, television sets, and the like.
A plasma display device displays full color by means of additive color mixing of so-called three primary colors (red, green, and blue). For displaying full color, a plasma display device is provided with phosphor layers that emit light in the three primary colors: red (R), green (G), and blue (B). Phosphor particles composing the phosphor layers are excited by ultraviolet light occurring in a discharge cell of the PDP, to generate visible light in each color.
Compounds used for the above-mentioned phosphors in each color include (YGd)BO3:Eu3+ and Y2O3:Eu3+ for emitting red light; Zn2SiO4:Mn2+ for green; and BaMgAl1.0O17:Eu2+ for blue. These phosphors, after given raw materials being mixed therewith, are produced with a solid-phase reaction by being fired at a temperature above 1,000° C. This method is disclosed in “Phosphor Handbook (in Japanese)” (p. 219 to p. 220, by Ohmsha, Ltd., 1991), for example.
The phosphor particles produced by firing, after being lightly crushed to the extent of breaking aggregated particles but not the crystals, are screened (average particle diameter for red and green: 2 μm to 5 μm, for blue: 3 μm to 10 μm) before use. The reason for lightly crushing and screening (classifying) phosphor particles is as follows. That is, methods of forming a phosphor layer in a PDP include screen-printing of pasted phosphor particles in each color, and an ink-jet method, in which the paste is discharged through a nozzle for applying. Large agglomerates are included in a phosphor unless the phosphor particles are classified after being lightly crushed, and thus unevenness in coating and clogging in the nozzle may occur when coating the paste with the phosphors. Therefore, phosphors classified after being lightly crushed are small in particle diameter and even in particle size distribution, thus yielding a more desirable coated surface.
An example method of producing a green phosphor made of Zn2SiO4:Mn is disclosed in “Phosphor Handbook (in Japanese)” (pp. 219-220, Ohmsha, Ltd., 1991). That is, SiO2 is blended in ZnO at the rate of 1.5ZnO/SiO2, which is larger than its stoichiometric ratio (2ZnO/SiO2), and then fired at 1,200° C. to 1,300° C. in the air (at one atmospheric pressure) to produce a green phosphor. In this case, the surface of the Zn2SiO4:Mn crystal is covered with an excessive amount of SiO2, and the phosphor surface is negatively charged.
The fact that a green phosphor in a PDP, negatively charged with a high level, degrades in its discharge characteristic, is disclosed in Japanese Patent Unexamined Publications No. H11-86735 and No. 2001-236893, for example. Further, it is known that ink-jet coating, in which coating is made continuously with ink for a negatively charged green phosphor through a fine-bore nozzle, causes clogging in the nozzle and unevenness in coating. These are because ethyl cellulose in the ink is in particular presumably resistant to being adsorbed in the surface of the negatively charged green phosphor.
Further, there is a problem in which a negatively charged phosphor causes ion collision of a negatively charged green phosphor with positive ions of Ne, CH-base, or H occurring while discharging, thus deteriorating the luminance of the phosphor.
Meanwhile, some methods are formulated such as laminate-coating a positively charged oxide for changing negative charge on the surface of Zn2SiO4:Mn to positive one, and mixing a positively charged green phosphor for apparently positive charge. However, it is problematic that laminate-coating oxide causes a low luminance, and applying two different kinds of phosphor with a different charge state tends to generate clogging and unevenness in coating. In addition, there is a method in which ZnO and SiO2 are blended in advance at the ratio of 2:1 or more (2/1 or more of Zn/Si in element ratio) when producing Zn2SiO4:Mn, and ZnO is scattered (sublimed) in advance while firing, using the vapor pressure of ZnO that is higher than that of SiO2, when firing the phosphor in the air or in nitrogen at one atmospheric pressure at 1,200° C. to 1,300° C. However, even in such a case, the proximity of the crystal surface results in rich SiO2 and is negatively charged by all means.
The present invention, in view of these problems, aims at preventing phosphor layers from deteriorating and at improving the luminance, life, and reliability of a PDP.